In order to talk to a programmer via ISP, the AVR chip needs to have a valid clock. The fuses control the source of the clock, and by error it is possible to set that source to something that doesn't exist (typically "external oscillator" when you meant "external crystal", or "extern crystal" when there isn't one connected.) At that point, the programmer can no longer talk to the chip.
You can usually recover from this by injecting a clock signal into the appropriate pin.
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=106325